Passenger aircrafts are designed with the aim to secure as much passenger seats as possible, in order to allow many passengers to be on board the aircrafts for commercial reasons, but increasing the number of passenger seats leads to increasing the number of lavatories (toilets) to be provided, so that limited spaces on the aircrafts must be utilized efficiently.
In response to such demands, a configuration is known where an interior of an aircraft is divided into an upper deck and a lower deck, wherein cabin modules are arranged on each of the upper deck and the lower deck, and the upper deck and the lower deck are designed to be connected to a staircase that is disposed separately, through which passengers are enabled to move up and down (refer to Patent Literature 1).
Similarly, a method is known where a toilet facility is arranged in a space below a floor of a passenger deck or in a space above a ceiling of the passenger deck of an aircraft, and a staircase is connected from the passenger deck to a toilet space so as to allow efficient use of a floor space of the passenger deck, and reduce the floor space used for installing the toilet facility in the aircraft (refer to Patent Literature 2).